Urethanes are conventionally produced by reacting alcohols with isocyanates. The characteristic of this substance class, i.e. of the urethanes, is the urethane functional group —O—CO—NH—. An alternative method for generating urethane groups is the reaction of a cyclic carbonate with a primary amine. The particular feature of this synthesis route to urethanes consists in the fact that it is possible to eliminate the use of isocyanates as raw materials. The person skilled in the art would refer to urethanes which are accessible via the last-mentioned synthesis route as “non-isocyanate urethanes” (NIUs).
Associative thickeners based on polyurethane are linear or branched, nonionic polymers with hydrophilic and hydrophobic segments. To an increasing extent, they supplement or substitute for the cellulose ethers and alkali-soluble polyacrylates traditionally used as thickeners in paints and coatings. Compared with these, the polyurethane thickeners have the following advantages: lower viscosity upon incorporation, lower spray tendency upon spreading, better color flow, higher shine as a result of less flocculation, lower sensitivity of the coatings to water, lower susceptibility to microbial attack, low intrinsic viscosity of the formulated material and good meterability resulting therefrom, high thickening effect in the dispersions to be thickened, lowest possible drop in viscosity of the thickened dispersions under shear, and approximately Newtonian flow behavior.
Polyurethane solutions or dispersions in water-dilutable aqueous or predominantly aqueous phase are referred to by the person skilled in the art as HEUR thickeners (the acronym HEUR is derived from “nonionic hydrophobically modified ethylene oxide urethane block copolymer”), and have already been used for a relatively long time in highly diverse fields of application for thickening water-based emulsion paints. The thickeners fundamentally described in U.S. Pat. No. 4,079,028 are composed of linear and/or branched polyethylene glycol blocks and hydrophobic segments which are generally linked together via urethane groups.
Although not wishing to be bound by any particular theory, the mechanism of action of such thickeners is assumed to be that the polyethylene glycol segments ensure the water compatibility, and the hydrophobic segments construct a viscosity-imparting three-dimensional molecular network via an association with one another and also with dispersed binder particles of the emulsion paint to be thickened therein.
Preferred hydrophobic starting materials are relatively long-chain, generally monofunctional alcohols, such as, for example, n-octanol, n-dodecanol, isotridecyl alcohol or isononylphenol. These alcohols are employed predominantly as they are, but may also be used in the form of their addition products with a few equivalents of ethylene oxide. The polyfunctional isocyanates used are generally difunctional, such as, for example, methylenebis(4-cyclohexyl) diisocyanate, m/p-tetramethylene-xylylene diisocyanate, hexamethylene diisocyanate, 2,4-tolylene diisocyanate, trimethylhexamethylene diisocyanate and 4/2,4′-diphenyl-methane diisocyanate. The polyethylene glycols used are generally likewise difunctional, and have molecular weights in the region of a few thousand daltons, for example 4500 or 10000 daltons.